The present invention relates to magnetic recording devices for recording information on recording media with use of magnetism, such as optomagnetic disk drives, hard disk drives, etc., and more particularly to a magnetic head drive circuit for use in the magnetic recording device. Stated more specifically, the present invention relates to improvements in the switching characteristics of a switch circuit for changing the direction of the current to be passed through the field coil of a magnetic head.
The magnetic recording device is adapted to convert information to a recording signal which is a digital voltage signal and to hold a recording medium magnetized in directions corresponding to the recording signal to record the information on the medium. The device comprises a magnetic head for setting up a magnetic field on the recording medium, and the magnetic head has a field coil for producing the magnetic field from current. The magnetic recording device further comprises a magnetic head drive circuit for energizing the field coil and changing the direction of the current for energizing the field coil based on the recording signal.
FIG. 4 is a diagram showing a conventional magnetic head drive circuit. The magnetic head drive circuit 100 comprises a control circuit 110 for producing a control signal based on a recording signal, and a switch circuit 120 for changing the direction of current for energizing a field coil 101 based on the control signal received from the control circuit 110. The switch circuit 120 comprises switch elements for turning on or off a current path based on the control signal received from the control circuit 110.
The switch elements to be used in the magnetic head drive circuit are preferably those which are short in switching time taken for a change of the current state to an on state or off state after the reception of the control signal from the control circuit. For this reason, FETs (field-effect transistors) are used specifically as illustrated.
There are two types of FETs: the N-channel type and the P-channel type. The Nch (channel) FET is such that the drain is connected to the high potential side and the source to the low potential side. The transistor conducts, permitting current to flow from the drain to the source, when the voltage between the gate and the source (hereinafter referred to as xe2x80x9cgate voltagexe2x80x9d) rises beyond a threshold voltage. When the gate voltage drops below the threshold voltage, the transistor is brought out of conduction to block the passage of current. On the other hand, the Pch FET is such that the source is connected to the high potential side, and the drain to the low potential side. The transistor conducts when the gate voltage drops below the threshold voltage, permitting current to flow from the source to the drain. When the gate voltage rises above the threshold voltage, the transistor is brought out of conduction to block the passage of current.
To obtain a stabilized gate voltage, it is desirable to use the Nch FET which is connected at the source to the low potential side in the case where the potential at the low potential side is stable. In the case where the high potential side is stable, the Pch FET is preferable to use which is connected to the high potential side at its source.
Accordingly, the switch elements connected to a power source terminal 102 of positive potential as shown in FIG. 4 are preferably Pch FETs, while the switch elements connected to a ground terminal 103 are preferably Nch FETs.
With reference to the magnetic head drive circuit of FIG. 4, the power source terminal 102 of positive potential Vd is connected to the sources S of first and second Pch FETs 121, 122. The drain D of the first Pch FET 121 is connected to one end of the field coil 101 and the drain of a first Nch FET 123. The drain of the second Pch FET 122 is connected to the other end of the field coil 101 and the drain of a second Nch FET 124. The sources of the first and second Nch FETs 123, 124 are connected to the ground terminal 103. In this way,the switch elements arranged on sides of an electric path surrounding a rectangle, the power from the power source is supplied between two nodes 125, 126 on one pair of opposed sides, and the field coil 101 is connected between two nodes 127, 128 on the other pair of opposed sides. The circuit thus constructed is generally termed a xe2x80x9cbridge circuit.xe2x80x9d
The control circuit 110 comprises amplifiers 111, 112 for amplifying recording signals and recording signals as inverted, respectively. Power is supplied to the amplifiers 111, 112 from the power source terminal 102. The first amplifier 111 receives the recording signal, amplifies voltage of high (H) level to Vd volts and feeds the resulting voltage to the first Pch and first Nch FETs 121, 123. The second amplifier 112 receives the recording signal as inverted, amplifies voltage of high (H) level to Vd and feeds the resulting voltage to the second Pch and second Nch FETs 122, 124.
When the control circuit 110 of the magnetic head drive circuit 100 thus constructed receives a recording signal of H level, the first amplifier 111 applies voltage of H level to the gates of the first Pch and first Nch FETs 121, 123, and the second amplifier 112 applies voltage of L level to the gates of the second Pch and second Nch FETs 122, 124. This brings the first Pch FET 121 and the second Nch FET 124 out of conduction, and the second Pch FET 122 and the first Nch FET 123 into conduction. Accordingly, current can be passed from the power source terminal 102 to the ground terminal 103 via the second Pch FET 122, the field coil 101 and the first Nch FET 123. The current flows through the field coil 101 in the direction of arrow A.
Next, when the control circuit 110 receives a recording signal of L level, the first amplifier 111 applies voltage of L level to the gates of the first Pch and first Nch FETs 121, 123, and the second amplifier 112 applies voltage of H level to the gates of the second Pch and second Nch FETs 122, 124. This brings the first Pch FET 121 and the second Nch FET 124 into conduction, and the second Pch FET 122 and the first Nch FET 123 out of conduction. Accordingly, current can be passed from the power source terminal 102 to the ground terminal 103 via the first Pch FET 121, the field coil 101 and the second Nch FET 124. The current flows through the field coil 101 in the direction of arrow B.
Thus, the direction of the current through the field coil 101 can be changed by the magnetic head drive circuit 110 of the construction described in accordance with the voltage level of the recording signal.
However the conventional construction has the following problem.
While the current (hereinafter referred to as xe2x80x9chead currentxe2x80x9d) flowing through the field coil 101 is given the desired value for the magnetic head to set up a magnetic field of desired intensity on the recording medium, it is desired that the period during which the magnetic field retains the desired intensity be longer. For this purpose, it is desirable that the switching FETs 121, 122, 123, 124 of the magnetic head drive circuit 100 can be changed over rapidly. However, the Pch FET is greater than the Nch FET in the time constant of charging or discharging at the gate. Accordingly, the period Tp taken for the Pch FETs 121, 122 to change the on-off state thereof after the reception of the control signal until the gate is charged or discharged past the threshold voltage for the change is longer than the corresponding period Tn taken for the Nch FETs. Accordingly, the head current will not become the desired value during the period T1 from the change of on-off state of the Nch FETs until the change of on-off state of the Pch FETs. As a result, the period TO becomes shorter during which the head current is given the desired value xc2x1I1.
An object of the present invention is to provide a magnetic head drive circuit wherein Nch FETs and Pch FETs are both used as switching FETs and which is prolonged in the period during which the magnetic field to be set up by a magnetic head on a recording medium is given the desired intensity.
The present invention relates to a magnetic head drive circuit comprising a control circuit for receiving a recording signal for recording data on a recording medium with use of magnetism and producing a control signal based on the recording signal, and a switch circuit for changing the direction of current to be passed through a field coil of a magnetic head based on the control signal received from the control circuit, the switch circuit comprising switching FETs for changing the current direction. Pch and Nch FETs are both used as the switching FETs of the switch circuit.
To fulfill the foregoing object, the magnetic head drive circuit of the invention is characterized in that the maximum value of the gate voltage to be applied to the gates of the switching FETs is lower than the voltage of a coil power source for supplying the current to be passed through the field coil and higher than the gate threshold voltage of the switching FETs.